- DIGITAL CAMERA SENSOR SIZES -

This article aims to address the question: how does your digital camera's sensor size
influence different types of photography? Your choice
of sensor size is analogous to choosing between 35 mm, medium format and
large format film cameras-- with a few notable differences unique to digital
technology. Much confusion often arises on this topic because there
are both so many different size options, and so many trade-offs relating to
depth of field, image noise, diffraction, cost and size/weight.

I have written this article after conducting my own research to decide
whether the new Canon EOS 5D is really an upgrade from the 20D for the
purposes of my photography. Background reading on this topic can be
found in the tutorial on
digital camera sensors.

OVERVIEW OF SENSOR SIZES

Sensor sizes currently have many possibilities, depending on their use,
price point and desired portability. The relative size for
many of these is shown below:

Canon's 1Ds/1DsMkII/5D and the Kodak DCS 14n are the most common full
frame sensors. Canon cameras such as the 300D/350D/10D/20D all have a
1.6X crop factor, whereas Nikon cameras such as the D70(s)/D100 have a 1.5X
crop factor. The above chart excludes the 1.3X crop factor, which is
used in Canon's 1D series cameras.

Camera phones and other compact cameras use sensor sizes in
the range of ~1/4" to 2/3". Olympus, Fuji and Kodak all teamed up to
create a standard 4/3 system, which has a 2X crop factor compared to 35 mm
film. Medium format and larger sensors exist,
however these are far less common and currently prohibitively expensive.
These will thus not be addressed here specifically, but the same principles still
apply.

CROP FACTOR & FOCAL LENGTH MULTIPLIER

The crop factor is the sensor's diagonal size compared to a full-frame 35 mm
sensor.
It is called this because when using a 35 mm lens, such a sensor
effectively crops out this much of the image at its exterior (due to its
limited size).

35 mm Full Frame Angle of View

One might initially think that throwing away image information is never
ideal, however it does have its advantages. Nearly all lenses are sharpest at their centers, while quality degrades progressively
toward to the edges. This means that a cropped sensor effectively
discards the lowest quality portions of the image, which is quite useful when using low
quality lenses (as these typically have the worst edge quality).

Uncropped Photograph

Center Crop

Corner Crop

On the other hand, this also means that one is carrying a much larger
lens than is necessary-- a factor particularly relevant to those carrying
their camera for extended periods of time (see section below).
Ideally, one would use nearly all image light transmitted from the lens, and this
lens would be of high enough quality that its change in sharpness would be
negligible towards its edges.

Additionally, the optical performance of wide angle lenses is rarely
as good as longer focal lengths. Since a cropped sensor is forced
to use a wider angle lens to produce the same angle of view as a larger
sensor, this can degrade quality. Smaller sensors also enlarge the
center region of the lens more, so its resolution limit is likely to be more
apparent for lower quality lenses. See the
tutorial on camera lens quality for more on this.

Similarly, the focal length multiplier relates the focal length of a lens used
on a
smaller format to a 35 mm lens producing an equivalent angle of view, and is equal to
the crop factor. This means that a 50 mm lens used on a sensor with a
1.6X crop factor would produce the same field of view as a 1.6 x 50 = 80 mm
lens on a 35 mm full frame sensor.

Focal Length Multiplier Calculator

Sensor Type:

Actual Lens Focal Length:

mm

Focal Length Multiplier
35 mm Equivalent
Focal Length

Be warned that both of these terms can be somewhat
misleading. The lens focal length does not change just because a lens
is used on a
different sized sensor-- just its angle of view. A 50 mm lens is
always a 50 mm lens, regardless of the sensor type. At the same time,
"crop factor" may not be appropriate to describe very small sensors
because the image is not necessarily cropped out (when using lenses designed
for that sensor).

LENS SIZE AND WEIGHT CONSIDERATIONS

Smaller sensors require lighter
lenses (for equivalent angle of view, zoom range, build quality and aperture
range). This difference may be critical for wildlife, hiking and
travel photography because all of these often utilize heavier lenses or
require carrying equipment for extended periods of time. The chart
below illustrates this trend for a selection of Canon telephoto lenses
typical in sport and wildlife photography:

An implication of this is that if one requires the subject to occupy the same fraction of the image on
a 35 mm camera as using a 200 mm f/2.8 lens on a camera with a
1.5X crop factor (requiring a 300 mm f/2.8 lens), one would have to carry
3.5X as much weight! This also ignores the size difference between the
two, which may be important if one does not want to draw attention in
public. Additionally, heavier lenses typically cost much more.

For SLR cameras, larger sensor sizes result in larger and clearer
viewfinder images, which can be especially helpful when manual focusing.
However, these will also be heavier and cost more because they require a
larger prism/pentamirror to transmit the light from the lens into the
viewfinder and towards your eye.

DEPTH OF FIELD REQUIREMENTS

As sensor size increases, the
depth of field
will decrease for a given aperture (when filling the frame with a subject of
the same size and distance). This is because larger sensors require
one to get closer to their subject, or to use a longer focal length in order
to fill the frame with that subject. This means that one has to
use progressively smaller aperture sizes in order to maintain the same depth
of field on larger sensors. The following calculator predicts
the required aperture and focal length in order to achieve the same depth of
field (while maintaining perspective).

Depth of Field Equivalents

Sensor #1

Selected aperture

Actual lens focal length

mm

Sensor #2

Required Focal Length (for same perspective)Required Aperture

As an example calculation, if one wanted to reproduce the same
perspective and depth of field on a full frame sensor as that attained
using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one
would need to use a 16 mm lens and an aperture of roughly f/18.
Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor,
this would produce a depth of field so shallow it would require an
aperture of 0.9 on a camera with a 1.6X crop factor-- not possible with
consumer lenses!

A shallower depth of field may be desirable for portraits because it improves
background blur, whereas a larger depth of field is desirable for landscape
photography. This is why compact cameras struggle to produce
significant background blur in portraits, while large format cameras
struggle to produce adequate depth of field in landscapes.

Note that the above calculator assumes that you have a lens on the new
sensor (#2) which can reproduce the same angle of view as on the original
sensor (#1). If you instead use the same lens, then the aperture
requirements remain the same (but you will have to get closer to your
subject). This option, however, also changes perspective.

INFLUENCE OF DIFFRACTION

Larger sensor sizes can use smaller apertures before the
diffraction airy
disk becomes
larger than the circle of confusion (determined by print size
and sharpness criteria). This is primarily because larger sensors do
not have to be enlarged as much in order to achieve the
same print size. As an example: one could theoretically use a digital
sensor as large as 8x10 inches, and so its image would not need to be
enlarged at all for a 8x10 inch print, whereas a 35 mm sensor would require
significant enlargement.

Use the following calculator to estimate when diffraction begins to
reduce sharpness. Note that this only shows when diffraction will be
visible when viewed onscreen at 100%-- whether this will be apparent in the
final print also depends on viewing distance and print size. To calculate
this as well, please visit:
diffraction limits and photography.

Diffraction Limited Aperture Estimator

Sensor Size

Resolution

Megapixels

Diffraction Limited Aperture

Keep in mind that the onset of diffraction is gradual, so apertures
slightly larger or smaller than the above diffraction limit will not all of
a sudden look better or worse, respectively. Furthermore, the above is
only a theoretical limit; actual results will also depend on lens
characteristics. The
following diagrams show the size of the airy disk (theoretical maximum
resolving ability) for two apertures against a grid representing pixel size:

Pixel Density Limits Resolution(Shallow DOF Requirement)

Airy Disk Limits Resolution(Deep DOF Requirement)

An important implication of the above results is that the
diffraction-limited pixel size increases for larger sensors (if the
depth of field requirements remain the same). This pixel size refers
to when the airy disk size becomes the limiting factor in total resolution--
not the pixel density. Further, the diffraction-limited depth of field
is constant for all sensor sizes. This factor may be critical when deciding on a
new camera for your intended use, because more pixels may not necessarily
provide more resolution (for your depth of field requirements). In
fact, more pixels could even harm image quality by increasing noise and
reducing dynamic range (next section).

PIXEL SIZE: NOISE LEVELS & DYNAMIC RANGE

Larger sensors generally also have larger pixels (although this is not
always the case), which give them the potential to produce lower
image noise and
have a higher dynamic range. Dynamic range describes the range of tones which
a sensor can capture below when a pixel becomes completely white, but yet
above when texture is indiscernible from background noise (near black).
Since larger pixels have a greater volume -- and thus a greater
range of photon capacity -- these generally have a higher dynamic range.

Note: cavities shown without color filters present

Further, larger pixels receive a greater flux of photons during a given
exposure time (at the same f-stop), so their light signal is much
stronger. For a given amount of background noise, this produces a
higher signal to noise ratio -- and thus a smoother looking photo.

Larger Pixels
(with a Larger Sensor)

Smaller Pixels
(with a Smaller Sensor)

This is not always the case
however, because the amount of background noise also depends on sensor
manufacturing process and how efficiently the camera extracts tonal
information from each pixel (without introducing additional noise). In
general though, the above trend holds true. Another aspect to consider
is that even if two sensors have the same apparent noise when viewed at
100%, the sensor with the higher pixel count will produce a cleaner looking
final print. This is because the noise gets enlarged less for the
higher pixel count sensor (for a given print size), therefore this
noise has a higher frequency and thus appears finer
grained.

COST OF PRODUCING DIGITAL SENSORS

The cost of a digital sensor rises dramatically as its area increases.
This means that a sensor with twice the area will cost more than twice as
much, so you are effectively paying more per unit "sensor real estate" as
you move to larger sizes.

Silicon Wafer(divided into small sensors)

Silicon Wafer(divided into large sensors)

One can understand this by looking at how manufacturers make their
digital sensors. Each sensor is cut from a larger sheet of silicon
material called a wafer, which may contain thousands of individual chips.
Each wafer is extremely expensive (thousands of dollars), therefore fewer
chips per wafer result in a much higher cost per chip. Furthermore,
the chance of an irreparable defect (too many hot pixels or otherwise) ending up in a given sensor
increases with sensor area, therefore the percentage of usable sensors
goes down with increasing sensor area (yield per wafer). Assuming
these factors (chips per wafer and yield) are most important, costs increase
proportional to the square of sensor area (a sensor 2X as big costs 4X as
much). Real-world manufacturing has a more complicated size versus
cost relationship, but this gives you an idea of skyrocketing costs.

This is not to say though that certain sized
sensors will always be prohibitively expensive; their price may eventually
drop, but the relative cost of a larger sensor is likely to remain
significantly more expensive (per unit area) when compared to some smaller size.

CONCLUSIONS: OVERALL IMAGE DETAIL & COMPETING FACTORS

Depth of field is much shallower for larger format sensors, however one
could
also use a smaller aperture before reaching the diffraction limit (for your
chosen print size and sharpness criteria). So which option has the
potential to produce the most detailed photo? Larger sensors (and
correspondingly higher pixel counts)
undoubtedly produce more detail if you can afford to sacrifice depth of
field. On the other hand, if you wish to maintain the same depth of
field, larger sensor sizes do not necessarily have a resolution advantage.
Further, the diffraction-limited depth of field is the same for all
sensor sizes. In other words, if one were to use the smallest
aperture before diffraction became significant, all sensor sizes would
produce the same depth of field-- even though the diffraction limited
aperture will be different.

Technical Notes:This result assumes that your pixel size is comparable to the size of the
diffraction limited airy disk for each sensor in question, and that each lens is
of comparable quality. Furthermore, the tilt lens feature is far more common in
larger format cameras-- allowing one to change the angle of the focal
plane and therefore increase the apparent depth of field.

Another important result is that if depth of field is the limiting
factor, the required exposure time increases with sensor size for the same sensitivity.
This factor is probably most relevant to macro and nightscape photography,
as these both may require a large depth of field and reasonably short exposure
time. Note that even if photos can be taken handheld in a
smaller format, those same photos may not necessarily be taken handheld in
the larger format.

On the other hand, exposure times may not
necessarily increase as much as one might initially assume because larger sensors
generally have lower noise (and can thus afford to use a higher sensitivity
ISO setting while maintaining similar perceived noise).

No matter what the pixel size, larger sensors unavoidably have more
light-gathering area. Theoretically, a larger sensor with
smaller pixels will still have lower apparent noise (for a given print size)
than a smaller sensor with larger pixels (and a resulting much lower total
pixel count). This is because noise in the higher resolution
camera gets enlarged less, even if it may look noisier at 100% on your
computer screen. Alternatively, one could conceivably average adjacent
pixels in the higher pixel count sensor (thereby reducing random noise)
while still achieving the resolution of the lower pixel count sensor.
This is why
images downsized for the web
and small prints look so noise-free.

Technical Notes:This
all assumes that differences in
microlens
effectiveness and pixel spacing are negligible for different sensor sizes.
If pixel spacing has to remain constant (due to read-out and other circuitry
on the chip), then higher pixel densities will result in less light
gathering area unless the microlenses can compensate for this loss.
Additionally, this ignores the impact of fixed pattern or dark current
noise, which may vary significantly depending on camera model and read-out
circuitry.

Overall: larger sensors generally provide more control and greater
artistic flexibility, but at the cost of requiring larger lenses and more
expensive equipment. This flexibility allows one to create a shallower depth of field
than possible with a smaller sensor (if desired), but yet still achieve a
comparable depth of field to a smaller sensor by using a higher ISO speed
and smaller aperture (or when using a tripod).